Apparatus and method for local chemical analyses at the surface of solid materials by spectroscopy of X photoelectrons

ABSTRACT

The apparatus for local chemical analyses at the surface of solid materials by spectroscopy of X photoelectrons comprising an ultravacuum analysis chamber 1 wherein is housed the sample to be analyzed 2 which is connected to a manipulator 3 located outside said chamber 1, an analyzer 4 in the vicinity of the sample and an electron source 5 emitting an electron beam 10 is characterized in that it comprises between the electron beam 10 and the sample 2 which is a bulky solid material, a microsource of X photons 6 placed as close as possible to said bulky sample 2.

The present invention relates to an apparatus and a method for localchemical analyses at the surface of solid materials by X rayphotoelectron spectroscopy.

X ray photoelectron spectroscopy (XPS) involves irradiating a samplelocated in an ultra high vacuum enclosure with x-ray photons photons andanalyzing the photoelectrons emitted by the sample. The energy spectrumof the photoelectrons is used to discover the chemical composition ofthe surface as well as the nature of the chemical bonds found in thesurface layers of atoms and molecules. Sources of x-ray photons includean x-ray tube of the:

type used in radiography or radiocrystallography, which is essentiallycomposed of an anode bombarded by an electron beam,

synchrotron radiation provided by some particle accelerators

and the source recommended by the J. CAZAUX method from Universite deREIMS wherein the sample or the sample support acts as an anode. In theCAZAUX method, one of the faces is bombarded by an electron beam and, ifthe sample and its support are thin enough, photoelectrons are generatedon the opposite face.

Using one of the above mentioned x-ray sources typically results in anarea of examination of approximately 10 mm².

Various techniques are known for reducing the area analyzed with thesex-ray sources.

One technique involves collimating the x-ray source, wherein theirradiated area is reduced. However the flux of x-ray photons iscorrespondingly decreased and the signal-to-noise ratio of the detectedsignals suffers.

Another method is to reduces the analyzed area by placing a device atthe analyzer input or otherwise reducing the field of observation. Onceagain the signal-to-noise ratio is reduced.

Reduction has been achieved by focusing the X rays with a grating.However, this reduction is also achieved at the expense of a significantloss of the signal-to-noise ratio, thus requiring the use of amulticollecting detector,

When using the CAZAUX method of generating x ray photons in a thin anodea focused electron beam reduces the area, however the thin sample mustbe integral with the anode.

At the present time, the CAZAUX method gives the best resolution, i.e.some microns, but it only applies to thin samples, thus considerablyrestricting its range of application. The CAZAUX method requires samplesof the order of 0,1 to 1 micron, and, it is obvious to those skilled inthe art that serious problems arise with samples of such a size. First,such samples are difficult to obtain additionally, the technique ofsurface analysis is extremely difficult to control with such thin samplesurfaces, the analyzed thickness of which may sometimes be of the orderof some atomic monolayers. It is also essential to clean the sample,which of course causes problems in itself.

When X rays are to be focused by a grating, the cost of the x-ray sourceand the monochromator is greater than 1,000,000 francs ($165,000).

The price of a conventional x-ray source with its supply is of the orderof 300,000 to 500,000 francs ($50,000 to $85,000).

The present invention provides an apparatus and a method for localchemical analyses at the surface of solid materials by X rayphotoelectron spectroscopy. Furthermore, there are no strict limitationsdue to the sample size which may range in size greatly and analyses canbe carried out on a surface, the diameter of which may vary from about100 microns to 1 millimeter, at relatively little expense.

The apparatus of the present invention enables local chemical analysesat the surface of solid materials by X ray photoelectron spectroscopyand the simultaneous observation of the analyzed area of the sample byscanning electron microscopy. This allows for noting the presence ofanomalous surface features that may affect the analysis.

More specifically, the present invention is an apparatus for localchemical analyses at the surface of solid materials by X-rayphotoelectron spectroscopy that utilizes:

an ultra high vacuum analysis chamber housing the sample to be analyzed.The sample is connected to a manipulator extending, and controlled fromoutside the ultra vacuum chamber,

An analyzer is positioned in the vicinity of the sample within the ultrahigh vacuum chamber and the apparatus is provided with

an electron source. Between the electron source and the sample, whichmay be a bulky solid material, a microsource of X ray photons isprovided as close as possible to the sample. The microsource consists ofa body containing an anode and a collimator wherein the relationshipbetween these components is such that the body blocks the incidentelectrons of the electron beam emitted by the electron source so thatthey are not detected after reflection on the sample. The anticathode isthick enough to stop most of the electrons of the electron beam butsufficiently thin to limit absorption of the X ray photons generated andthe collimator reduces the emission angle of the photons and masks theemissive face of the anode with respect to the analyzer, as well as theinner cylinder of the collimator. Photoelectrons are emitted by the areaof the sample subject to the X-ray photons and received by the analyzer.

The present invention also teaches a method for local chemical analysesat the surface of solid materials by X ray photoelectron spectroscopy.

The solid sample to be analysed, connected to a manipulator, is fittedinto an ultra high vacuum analysis chamber

and an analyzer is put in the vicinity of said sample.

An electron beam from an electron source, is directed towards the samplewithin the ultra high vacuum chamber, however:

interposed between the electron beam and the sample, as close aspossible to said sample, is an X photon microsource. The X-ray photonmicrosource of a body containing an anticathode and a collimator, therelationship between these components is such that the body blocks theincident electrons of the electron beam so that they are not detectedafter reflection on the sample, the anticathode is thick enough to stopmost of the electrons of the electron beam but sufficiently thin tolimit absorption of the X-photons generated and the collimator reducesthe emission angle of the photons and masks the emissive face ofanticathode with respect to the analyzer, as well as the inner cylinderof the collimator. Photoelectrons are emitted by the sample being as aresult of the impact of X-ray photons and are received by the analyzer.

The present invention also includes features that may be takenseparately or in combinations.

It is preferable for the body of the X photon microsource to consist ofthree staged cylinders, the diameter of the first cylinder beingrelatively large compared with the diameter of the electron beam.

In the invention the anticathode is integral with the cylinder ofsmallest diameter of the body of X photon microsource and consists of athin metal film.

The thickness of the thin metal film is in the range of 1 micron to 50microns and

the metal film is selected from a metal group consisting of aluminum,magnesium, chromium, copper and the like.

The analyzed area of the sample is to have a diameter of about 100microns and

the sample is to be inclined at an angle of about 45° to the axis of theelectron beam.

The wall thickness of the body of the X photon microsource is to beabove 20 microns.

The collimator is a component extending the body of the X photonmicrosource and may be fastened to the latter by welding or by fittingin and being held in position by friction.

The X photon microsource may be operated from outside of the ultravacuumanalysis chamber by a manipulator;

The invention also contemplates focusing and scanning the electron beam,a secondary electron detector and the electronic components required forobtaining an electronic image of the microsource and the sample surface.

Various advantages and features of the present invention will beapparent from the detailed description below by reference to theaccompanying drawings in which:

FIG. 1 is a flow sheet of the invention.

FIG. 1A illustrates a particular embodiment of the invention.

FIG. 2 illustrates a flow sheet of the apparatus according to theinvention.

FIG. 3 shows a detail of a X photon microsource.

FIG. 4 illustrates a first embodiment of a X photon microsourceaccording to the invention.

FIG. 5 illustrates a second embodiment of a X photon microsourceaccording to the invention.

FIG. 6 is a cross-sectional view taken on line VII--VII of FIG. 7.

FIG. 7 is a cross-sectional view taken on line VI--VI of FIG. 6.

FIGS. 8 and 8 illustrate experimental results; on these figures, thephotoelectron energy in electron-volts is plotted as abscissa and thecounts per second as ordinate.

In the accompanying drawings where the same numerals designate similarparts, the apparatus for local chemical analyses at the surface of solidmaterials by X photoelectron spectroscopy is designated, as a whole, by22 (FIG. 2).

The apparatus 22 comprises, in a known manner, an electron source 5emitting an electron beam 10, an ultravacuum analysis chamber 1 in whichis housed the sample to be analyzed 2 which is connected to amanipulator 3 located outside said chamber 1 and an analyzer 4. Amicrosource of X photons 6 is interposed between the electron beam 10and the sample 2 as close as possible to said sample 2.

The X photon microsource 6 of the present invention consists of threecomponents, a body 7, a thin metal film or anticathode 8 and acollimator 9.

The body 7 of the X photon microsource 6 comprises three stagedcylinders 13, 14, 15 in order to trap the incident electrons and theelectrons reemitted by the anticathode 8. The diameter of the firstcylinder 13 is relatively large with respect to that of the electronbeam 10 to capture the background electrons accompanying the electronbeam 10. These electrons essentially result from interactions ofelectron beam 10 with the diaphragms of the electron source 5 orelectron gun. The general shape resulting from these staged cylinders13, 14, 15 allows to come close to the surface of a sample 2 inclined atan angle of about 45°.

X photons are generated in the metal film or anticathode 8. Thematerials used are those generally used for making anticathodes, thatis, aluminum, magnesium, chromium, copper and similar metals. Thethickness of the metal film 8 should be such that all incident electronsare stopped, but it should be the smallest possible in order to minimiseabsorption of the X photons before exiting the anticathode 8. Forinstance, a thin aluminum film of 3 microns is suitable for an electronbeam 10 of 15 kV. Concerning the thin film, the size can be varied inthe range of 1 to 50 microns. The structure of the anticathode 8according to the present invention assures, on one hand a sufficientphoton flux, on the other hand that the emissive surface, that is theend part, is not facing the analyzer; as a matter of fact, it isessential that the anticathode 8 should be as close as possible to thesample 2 and is situated in order not to mask the analyzed area withrespect to analyzer 4; thus, the closer is the anticathode 8 to thesample 2, the smaller is the surface to be analyzed.

The collimator 9 is the third component of the X photoelectronmicrosource 6 of the apparatus according to the invention. The primaryfunction of the collimator 9 is to reduce the irradiated surface ofsample 2 in order to improve the spatial analysis resolution. Theelectron beam 10 being focused at centre of the anticathode 8 (point 0,FIGS. 6 and 7), the irradiated surface 19 of sample 2 is then defined bythe closed curve A F B E A (FIG. 7) obtained by drawing straight linesfrom point 0 and passing through the lower end of the collimator 9 (incase the electron beam 10 is not strictly ponctual, the irradiatedsurface 19 will be slightly superior to that previously defined. Thebroadening due to this effect can be inferior to a 10% increase of theirradiated surface).

The end part of the collimator 9 has been designed in such a way that,on one hand the collimator 9 does not mask, towards the analyzer 4, theirradiated surface 19 of sample 2, on the other hand the inner cylinderof collimator 9 and the anticathode 8 are not facing analyzer 4. As amatter of fact, these components being irradiated by X photons, theyemit photoelectrons 12; a background signal would result if they weredetected by analyzer 4. Therefore, the collimator 9 ends in two inclinedplanes P₁ and P₂ (FIG. 6) tangent to the "acceptance" cone of analyzer4, the positions of which can be determined as follows:

Let us consider the case the electron beam 10 is vertical, the analyzeraxis 4 is horizontal and the sample 2 is inclined at 45° (FIG. 6). Theradius R of collimator 9 is selected on grounds of mechanicalfeasibility and the angle alpha is determined on grounds of X photonbeam 11 intensity. Both parameters R and alpha set the position of pointC. The plane P₂ inclined at an angle of -(θ/2) with respect to thehorizontal is drawn through the point C, θ/2 is the half-angle of"acceptance" of analyzer 4. The intersection of that plane with cylinder2R gives the point D. A plane set at an angle of (θ/2) with respect tothe horizontal is drawn through the point D, intersection of this planewith the straight line OC gives the point A. The point A being a pointof sample 2, the position of the sample with respect to the X photonmicrosource 6 will therefore be set. The following values can beinferred from above mentioned construction: ##EQU1##

Thus, according to the geometry illustrated in the FIGS. 6 and 7 andwith the values R=25 microns, α=25°, θ/2=30°, one obtains the valueshereinafter: OH₁ =53 microns, OH₂ =82 microns, OH₄ =200 microns, AB=200microns and EF=150 microns.

In the X photon microsource 6 of the invention, the collimator 9 is acomponent extending the body 7 which is fastened to the latter by anyappropriate means. By way of example, the collimator 9 is welded to thebody 7 as indicated at 16 or is fastened by fitting in as indicated at17 in the FIGS. 4 and 5. The effect of the body 7 of X photonmicrosource 6 is to block the incident electrons so that they are notdetected after reflection on sample 2. The walls of the body 7 must besufficiently thick and have a thickness above 20 microns in order tostop the photons emitted towards the walls of body 7.

The X photo microsource 6 is operated by a manipulator 18. Themanipulator 18 permits displacement of the anticathode 8 which is insidethe ultravacuum chamber 1. The link between the manipulator 18 and theanticathode 8 provides electrical insulation of the anticathode 6 andinforms about the intensity of the electron beam 10 and therefore aboutof the intensity of X photon beam. In the prior art, it has already beensuggested to put a X ray source in an ultravacuum enclosure but, in thiscase, the anticathode is bulky and must be located very remote fromsample. Because the source is very remote from the sample, a very highpower, generally of the order of 150 watts is needed, which involvescooling of the anticathode by a fluid. As a result, the irradiatedsurface of the sample is of the order of 10 mm² and a local analysiscannot be obtained.

One of the features of the present invention lies in that the samplesize is not limited and may have any value. Furthermore, the localanalysis achieved with the apparatus and the method according to thepresent invention ensures an analysis range of sample in the range of100 microns which cannot presently be obtained with the prior artdevices when bulky solid materials are used. With the prior art devices,this analysis range requires a focusing system of X rays.

The FIGS. 8 and 9 show the spectra of a sample of silver and copper witha beam energy of 40 KeV and an intensity of 0.7 μA, the energy width ofthe analyzer window being 3 eV, the data collecting time for the copperspectrum is 3 hours and that of the silver 11/2 hours.

The counting rate corresponding to the highest peak of the copperspectrum equals 7000 c/s. From this result, it can be seen that thecounting rate is 10 times higher than that given by a conventional Xsource collimated on 1 mm². By means of the apparatus and the methodaccording to the present invention, one obtains on a surface of 100microns diameter a counting rate equivalent to that obtained on asurface of 10 mm diameter with a conventional X source according to theprior art.

Therefore, those skilled in the art will appreciate the significantadvantages gained in practising the present invention such as the:

a moderate price of the apparatus and

the ability to use a specimen which is not limited as to its size.

Other advantages include the ability to analyze a surface area on theorder of 100 microns diameter and

the ability to localize the analysis by scanning electron microscopy.

The invention is not limited to the embodiment shown and described inmore details and various modifications can be made without departingfrom its scope.

I claim:
 1. Apparatus for local chemical analyses at a surface of asolid material by X-ray photoelectron spectroscopy, comprising:(a) anultra high vacuum analysis chamber for housing a sample to be analyzed;(b) support means for supporting said sample, said support meansextending outside said chamber; (c) an analyzer positioned within saidultra high vacuum analysis chamber in the vicinity of the sample; (d) anelectron source capable of emitting an electron beam; and (e) amicrosource of X-ray photons positioned between said electron source andsaid sample along the path of said electron beam provided as close aspossible to said solid sample, comprising: (i) a body comprising threestaged cylinders, the diameter of the first cylinder being relativelylarge compared with that of the electron beam; (ii) an anode containedwithin said body; and (iii) a collimator, wherein said body blocksincident electrons of said electron beam so that said electrons are notdetected after reflection on said sample, said anode being thick enoughto stop most of the electrons of said electron beam but sufficientlythin to limit absorption of X-ray photons generated and said collimatorreduces an emission angle of the photons and masks an emissive face ofsaid anode with respect to the analyzer, as well as an inner cylinder ofthe colimator, photoelectrons being emitted by the sample being receivedby said analyzer.
 2. Apparatus either of claims 1, wherein theanticathode (8) is integral with the cylinder of smallest diameter (15)of the body (7) of the microsource of X photons (6) and consists of athin metal film.
 3. Apparatus as claimed in claim 1 wherein thethickness of the thin metal film (8) is in the range of 1 micron to 50microns.
 4. Apparatus as claimed in claim 1 wherein the metal film (8)is selected from a metal group consisting of aluminum, magnesiumchromium and copper.
 5. Apparatus as claimed in claim 4, wherein themetal film is a metal selected from a group comprising aluminum,magnesium and chromium.
 6. Apparatus as claimed in claim 1, wherein theanalyzed area of sample (2) has a diameter (19) of about 100 microns. 7.Apparatus as claimed in claim 1, wherein the sample (2) is inclined atan angle of about 45°.
 8. Apparatus as claimed in claim 1, wherein thewall thickness of the body (7) of the microsource of X photons (6) isabove 20 microns.
 9. Apparatus as claimed in claim 1, wherein thecollimator (9) is a component extending the body (7) of the microsourceof X photons (6) which is fastened to the latter by welding (16). 10.Apparatus as claimed in claim 1, wherein the collimator (9) is acomponent extending the body (7) of the microsource of X photons (6)which is fastened to the latter by fitting in (17).
 11. Apparatus asclaimed in claim 1, wherein the microsource of X photons (6) is operatedfrom outside of the ultravacuum analysis chamber (1) by a manipulator(18).
 12. Apparatus as claimed in claim 1, wherein it comprises a device(23) for focusing and scanning the electron beam (10), a secondaryelectron detector (21) and the electronic components required forobtaining an electronic image of the microsource (6) and the surface ofsample (2).
 13. Apparatus as claimed in claim 1, wherein the collimatorend nearest the sample comprises two surfaces defining an angle havingan adjacent supplementary angle θ said angle θ bisected by the an axison which said analyzer is aligned.
 14. Method for local chemicalanalyses at a surface of solid materials by spectroscopy of X-rayphotoelectrons, wherein:(a) a solid sample to be analyzed, connected toa manipulator is fitted into an ultravacuum analysis chamber; (b) ananalyzer is placed within the ultra high vacuum analysis chamber in thevicinity of said sample; (c) scanning the sample with an electron beamfrom an electron source and (d) interposing between the electron beamand the sample, as close as possible to said sample, a microsource ofX-ray photons comprising a body containing an anode and a collimatorsuch that the body blocks the incident electrons of the electron beam sothat they are not detected after reflection on the sample, the anodethick enough to stop most of the electrons of said electron beam butsufficiently thin to limit absorption of X-ray photons generated and thecollimator reducing an emission angle of the photons and masking anemissive face of said anode with respect to the analyzer, as well as aninner cylinder of the collimator, photoelectrons emitted by the samplebeing received by said analyzer.
 15. Apparatus for local analysis of asurface of a solid material by X-ray photoelectron spectroscopy,comprising:(a) an ultra high vacuum analysis chamber; (b) a firstsupport for holding a sample within said ultra high vacuum analysischamber; (c) an electron source for introducing an electron beam intosaid ultra high vacuum analysis chamber; (d) a focusing means forfocusing said electron beam; (e) a microsource of X-ray photons,positioned between said electron source and said sample along the pathof said electron beam, comprising: (i) a three staged, cylindrical body;(ii) an anode, integral with said body, of metal film, and (iii) acollimator with angled faces at an output end; (f) a second support fororienting said microsource within said ultravacuum analysis chamber; and(g) a device for receiving photoelectron emissions of said sample, saiddevice in close proximity to said sample within said ultra high vacuumanalysis chamber.
 16. An apparatus as claimed in claim 15, wherein saidanode is of a thickness within the range of 1 to 50 microns and themetal is selected from a group consisting of aluminum, magnesium andchromium.
 17. An apparatus as claimed in claim 15 further comprising asecondary electron detector to enable electronic imaging of saidmicrosource and said sample.
 18. An apparatus as claimed in claim 15,wherein said sample is positioned so the surface analyzed is at a 45°angle with respect to said electron beam.
 19. An apparatus as claimed inclaim 18, wherein said device for receiving photoelectron emissions isaligned on an axis that bisects an adjacent supplementary angle to anangle defined by said angled faces of the output end of said collimator.